This demo makes use of CSS masking which is not very widely supported yet. At this time, Chrome and Mobile Safari appear to be the only browsers that do.
If you're not viewing this in Chrome or Mobile Safari the animations should still function properly, but the images that are supposed to be masked will not be. Wider adoption of CSS masking should be coming in the near future.
This is most definitely experimental. That means things will likely go haywire. I've seen animations not start, images dissappear, frequent crashes on Chrome and Safari for iOS, and all sorts of other weirdness. It may also set your computer on fire.
(Watch your volume, there's audio)